Soak the clay pot (top and bottom) according to manufacturer's directions.

Wash the spinach, then put the wet leaves in a large saucepan. Cover and cook over high heat, shaking the pan often, for about 5 minutes, or until the leaves have shrunk. Drain well, then place the spinach in the soaked clay pot.

Melt the butter or margarine in the pan. Add the garlic and ginger root, and cook for 3 minutes, stirring, then add the cumin, cardamom, coriander and turmeric and cook for 2 minutes. Remove from the heat and stir in the yogurt and season well with salt and pepper.

Arrange the fish fillets on top of the spinach, then spoon the spice mixture over. Cover the pot and place into the cold oven. Set the oven at 425 degrees. Bake for about 40 minutes or until the fish is just firm. Top with the tomatoes and chopped cilantro and serve garnished with lemon wedges.